It is known to use heat exchangers for warming a wide variety of fluids, including physiological fluids. U.S. Pat. No. 4,759,749 shows a heat exchanger in combination with a fluid supply system for circulating a warming fluid through a central tube of the heat exchanger. A physiological fluid is supplied to the space between the inner and outer tubes and is heated by contact with the warm inner tube.
A problem with fluid warming systems of this type arises when the flow rates are small, i.e., in the range of 100 ml/min. or less. At these rates, the physiological fluid leaving the heat exchanger cools in the delivery line between the heat exchanger and the patient. This problem worsens when the delivery line is of substantial length.
A heat exchanger having a central flexible tube and an outer flexible tube is known. The warming fluid is introduced at one end of the outer tube and flows out of an opening at the other end of the outer tube. This system is difficult to manufacture and presents several human engineering problems.
Heat exchangers having two concentric tubes formed by extrusion are also known. For example, U.S. Pat. No. 3,976,129 (Silver) shows an inner tube supported within an outer tube by fins, the structure being extruded from a metal, such as copper or aluminum.